Depends where it's leaking, but basically you fix it the same way you fix any coolant leak.
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Generally, its a hose that is leaking. You find the leaking hose, and you replace it, being sure to properly treat the ends with anti-leak compound. If its the radiator, you need to replace that. If its the intake manifold or head, you are getting into big bucks to replace the gasket involved.
You have to determine the source of the leak then decide on how to fix it.
You will have to locate the source of the leak before you can fix it. Re-post with your findings.
depends where the leak is, and how bad it is.
antifreeze leaking into the oil pan
You need to have your antifreeze up to par and if it is a leak you are worried about until you get somewhere to have it fixed there are quick fix solutions such as Barr's Leak that you can use it will seal the leak temporarily to get you to where you are going. Do not ever open your radiator cap if your car has been running - only open it when it is cold.
Pretty much going to have to replace it. Because of the pressure involved, there is no easy fix.
Pressure test system to determine where leak orginates to determine fix
Find source and fix it.
Replace the thermostat gasket.
Have cooling system pressure tested to determine origination of leak
by getting a Bars Leakradiator leak at walmart
The car will over heat.